At swimming pools, it boils down to this: clean tiles, or sun-protected skin. "Suntan oil accumulates on the surface of the pool, and makes a line on the tile, like a bathtub ring, that has to be removed," said Clevo Wheeling, sanitarian supervisor with the Fairfax County Health Department.

As a result, Fairfax and other jurisdictions have ordinances requiring a shower before swimming. At some pools, wearing T-shirts while swimming is also forbidden, because fibers can clog the filter system.

"Showers keep the quality of the water healthy so people can enjoy swimming," said Merni Fitzgerald, public information officer for the Fairfax County Park Authority.

But dermatologists believe people can be sunburned in water as easily as out, and studies have shown that a severe sunburn early in life can lead to skin cancer later. That medical view is worrying some local parents, who think the rules should be changed.

"It appears that the filter system is being protected at the expense of our current and future health, and that of our children," said Mary Alan Beckley, whose two children use the Lake Fairfax County Park pool in Reston three times a week while attending a day care program.

Beckley said she has instructed the day care counselors to continue putting sunscreen on her two children. "But it is not being done in accordance with the pool rules," she said.

There is no difference between swimming without sunscreen and walking around without sunscreen, said Dr. Martin Braun III, a District dermatologist and skin cancer surgeon. People will sustain the same type of burn in the water that they would if they were out in the sun, he said.

In the District, regulations governing pool use also require an initial shower. Although officials in Montgomery County highly recommend that people shower first, there is no ordinance to mandate it.